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BiologyThese birds return to breeding sites in late March to early April. Their breeding system is a mixture of monogamy and polygamy, and they establish breeding territories along a stretch of river by mid-April (5). Unusually amongst ducks, breeding trios of one male and two females are sometimes formed; breeding trios like these comprise up to 20 percent of breeding populations in Far East Russia (7). Clutches of four to twelve eggs are laid from the second half of April and throughout much of May. By early June, males leave the breeding grounds, while the females remain to incubate their eggs for 31 to 35 days. Nests are established in tree holes up to 18 metres above the ground, lined with down. Normally one clutch is laid per year, but if the first is destroyed a replacement may be laid. Broods hatch from May to June and most chicks fledge in the last ten days of August, at around eight weeks of age. In September and October, the birds migrate for the winter (5). The scaly-sided merganser usually forages in small groups of up to three birds and feeds on small fish as well as insect larvae, shrimps, crayfish and beetles, taken from the river (5).